|First Appearance||Chapter 1 (Manga)|
Ajin Part 1: Shōdō (Film)
Episode 1 (TV Anime)
In the beginning of the manga, her face was never clearly seen for unknown reasons. She has mid-long dark brown hair with a parting slightly to the middle. Her face is revealed in chapter 44. In the anime, she has brown eyes and wears a sweater.
Not much is known about her personality in the beginning of the story. With the little appearance she made in the manga, it can be assumed she is very logical and quite cold-hearted; comparing a dead pup to broken merchandise that she should get a refund for. In chapter 44, it is suggested that Kei inherits her cold and logical personality.
In the anime, she seemed a bit less so as the incident with the dog has not been mentioned so far. She asked Kei about his grades and how he is going on, saying she only expects good results, with Kei telling her not to worry.
It is known that her husband used to be a doctor and has died, leaving her alone ever since. Even though she claims to be a logical person, she states that the emotional side of her husband made her fall in love with him.
She seems distant even to her children. Even so, she loves them.
Kei is her son. It is unknown what kind of relationship they have; it can be speculated that her influence did have an effect on Kei, as when she told him to stop being friends with the son of a criminal, he immediately obeyed as to not get his reputation ruined. This could hint that Kei did have a high regard to what his mother said before everything happened, as he even now has the same cold, rational demeanor.
In the anime, their relationship is slightly different, adding an extra scene where she asks Kei about his grades and that she doesn't expect anything bad. Kei tells her not to worry, in contrast with the manga, where his mother seems to be absent for most of his life.
However, all of this relationship is strained when she discovered that her son is an Ajin. She, along with Eriko, believe that Ajin's don't deserve anyone's pity, and basically disowned him after this. It is unknown what her intentions really are, and if she actually cares for her children.
Notwithstanding, it is shown that Kei may not completely hold resentment towards her. In chapter 43, after parting with Kou and the rest of society, he decides to call her, possibly to bid his goodbye.
In chapter 44, she talks with her son over the phone. It is revealed that she actually cares for him, but she is not in the position to be involved in anything he does. She also learns that he wants to 'disappear'. However, she suspects that he 'will not run'.
Her daughter. Eriko views both her mother and brother to be too cold and rational, and seems to be hurt by her words when she talks about the dog as it was something materialistic and easily replaceable; a mere deficient item, rather than a living being. Ms. Nagai seems to somewhat neglect both of her children, showing little care about them; both when Kei ran away, and with Eriko's illness. She hears Eriko's opinion of Kei in chapter 44 and reprimands her that her 'cold' brother is just logical and she should stop bashing him.
- Her name, face, and personality in manga were up to interpretation before they were all revealed in chapter 44.
- In the anime, as well as the start of the manga, she is depicted as being much younger than in the recent manga chapters. It is most likely due to the artist's art progression, as well as the anime having no reference for her face as it wasn't revealed at that point in the manga.